The present invention relates to adhesives, and, more particularly, to an epoxy adhesive that is curable at ambient temperature but has high elevated temperature strength.
One of the most important parts of almost all manufacturing operations is the joining of structures that are separately prepared and then joined together into larger pieces, which themselves may be joined to other structures or may be the final assembled parts. Joining of structures is a critical operation, because subsequent failures may occur at the locations where structures are joined, or because there may be special requirements to be met at the interface between two structures. The surface along which two structures are joined must therefore be strong, resistant to failure by many different mechanisms such as fatigue and corrosion, and additionally must sometimes meet other requirements.
Structures used in aircraft and space vehicles are often subjected to some of the greatest demands placed upon any structures, since excellent physical properties must be coupled with low weight. Most structures have, in the past, been joined using mechanical fasteners such as rivets, bolts or screws, or by metallurgical bonding techniques such as welding, brazing or soldering. More recently, the properties of adhesives have improved so that many aerospace structures can be joined by adhesives. Adhesively bonded structures have some important structural advantages over structures bonded with fasteners, since the bonded structures may actually be stronger because the surface loading is distributed over larger areas. Because the loads are distributed, structural failure such as by fatigue cracks initiated at stres concentrations around fasteners is greatly reduced.
Aircraft and spacecraft often experience elevated temperatures during their operating lives, and there has been a continuing effort to develop adhesives having excellent strength at elevated temperatures. It has been found that existing adhesives that achieve excellent elevated temperature strength also require hardening or curing at elevated temperatures. Adhesives which reach their full strength in ambient temperature curing do not have the desired levels of strength at elevated temperature. A further requirement is that the adhesive produce acceptably low levels of particles that escape to the surrounding atmosphere or other environment during and after curing, so that particles are not present to damage or contaminate other parts of the structures or systems.
The inability to achieve excellent elevated temperature strength in an adhesive curable to full strength at ambient temperature makes fabrication of certain structures impossible using adhesive joining techniques. For example, large parts of the structure of a spacecraft may desirably be joined by adhesive bonding. These structures are subjected to elevated temperatures of about 250 degrees Fahrenheit during their operating lives, and therefore the adhesives must have acceptable strength levels at that temperature. The entire structures cannot be placed into an oven for elevated temperature curing of adhesives, since some parts of the spacecraft cannot stand the elevated temperatures. These latter parts may be kept cool during the operating cycle when the other parts are subjected to elevated temperatures, but could not be cooled effectively during a curing operation. Existing adhesives that cure at ambient temperature do not have sufficient strength at the required elevated temperature. Consequently, it has not been possible to utilize adhesive bonding to fabricate such structures, because no adhesive has been known that has the required elevated temperature strength coupled with the ability to develop full strength in ambient temperature curing. The ability to develop elevated temperature strength in an adhesive that cures at ambient temperature is also necessary to making field repairs, where no curing ovens are available.
There exists a need for a curable adhesive that can be cured at ambient temperature, has excellent strength at an elevated temperature of about 250 degrees Fahrenheit, and has low particle outgassing during curing and service. Good ductility and toughness of the adhesive are also important characteristics. The adhesive must also have a paste consistency prior to curing that allows its application to the structures being joined, and be readily applicable to the parts to be bonded. The present invention fulfills this need, and further provides related advantages.